1. Field of the Invention
The present invention relates to a lithium-intercalated compound, electrode structure, methods for producing them and lithium secondary battery, more particularly to a lithium-intercalated compound of structure having high capacity and high charge-discharge efficiency; an electrode structure composed of the lithium-intercalated compound of specific structure; a lithium secondary battery, and methods for producing the lithium-intercalated compound material and electrode structure.
2. Related Background Art
Global warming is anticipated, because of the greenhouse effect of CO2 which has been increasingly released into the air. Therefore, construction of new thermal power stations, which release large quantities of CO2, has become increasingly difficult. Devices for nighttime power have been adopted by private houses, to effectively utilize power produced by utilities.
Concept of load leveling has been proposed to use power supplied from a secondary battery in the daytime during which power is consumed more. Secondary batteries of high energy density have been increasingly in demand as the drivers for electric vehicles which release no pollutants. Compact, light, functional secondary batteries are also urgently needed as the power sources for portable devices, such as notebook personal computers, word processors, video cameras, and portable telephones.
The concept of compact, light, functional secondary battery was originally introduced in 1970 (Journal of the Electrochemical Society, 117, 222, 1970), and used a lithium-graphite intercalated compound as the anode for a secondary battery. As the compact, light, functional secondary battery, the rocking chair type secondary battery (the so-called lithium-ion battery) has been developed and commercialized since then. It uses, e.g., carbon (including graphite) as the anode active material and intercalated compound with the lithium ion in-between as the cathode active material. The lithium-ion battery uses the anode of carbon as the host material with lithium as the guest intercalated in-between, to control growth of dendrite during the charging step, thereby increasing charge-discharge cycles.
The lithium-ion battery with carbon as the anode active material, although long in cycle life, has a lower energy density than the lithium battery which uses metallic lithium itself as the anode active material.
Therefore, carbon material of higher capacity has been extensively studied and developed for the anode of the lithium-ion battery. It is also essential to develop cathode materials of higher capacity, in addition to anode materials, in order to realize secondary batteries of higher energy density. At present, transition metal oxides with lithium intercalated in-between are mainly used as the cathode materials. However, discharge capacity of these batteries is limited to 40 to 60% of the theoretical capacity. Therefore, development of cathodes of higher capacity, while having practical cycle life, has been strongly demanded for lithium secondary batteries, including the xe2x80x9clithium-ion batteryxe2x80x9d which uses lithium ions as the guest for charge/discharge reactions.
Techniques for gel electrolytes have been also extensively developed to prevent electrolyte leakage and facilitate production of batteries of good resistance to overcharge and various shapes by solidifying the electrolytes. Development of electrode materials suited for gel electrolytes, i.e., those forming good interfaces with the gel electrolyte, is also demanded.
The inventors of the present invention have already proposed hollow, porous structures for the active materials of long cycle life and high charge-discharge efficiency (Japanese Patent Application Laid-Open No. 8-321300), and also have been developing the active materials suited for gel-electrolyte batteries of higher performance.
It is an object of the present invention to provide a lithium-intercalated compound material of structure to have high capacity and high charge-discharge efficiency and suited for gel-electrolyte batteries, for lithium secondary batteries which use the intercalation/deintercalation reactions of the lithium ions for the charge-discharge reactions; an electrode structure composed of the lithium-intercalated compound material; and a lithium secondary battery of high capacity and high charge-discharge efficiency.
It is another object of the present invention to provide methods for producing the lithium-intercalated compound material for the electrode and electrode structure for the above lithium secondary battery.
The inventors of the present invention have found that a lithium secondary battery has high capacity, high charge-discharge efficiency and long serviceability, when a metal oxide of porous structure provided with ordered pores and capable of intercalating or deintercalating the lithium ions at least by the electrochemical reactions is used as the electrode material (electrode active material).